Thin films of crystalline material are desirable for a variety of solid state device applications. The solid state devices may exploit the electronic, optical, or other properties of the thin film materials. For example, optical wave retarder plates may be fabricated from ultra-thin films of single-crystal metal oxides exploiting the birefringence properties of the metal oxides. (See e.g., co-assigned Radojevic et al. U.S. Pat. No. 6,641,662 (“the '662 patent”), which is hereby incorporated by reference herein in its entirety). The desired thin films may have microscopic or mesoscopic thickness dimensions according to their intended use. The thin films may be free standing or supported on a substrate. In some material systems (e.g., in some III-V compound semiconductor systems) where the crystallographic properties of the desired thin films and the substrate material are compatible, the thin films can be formed by atomic layer-by-layer growth (i.e. epitaxial growth) on the substrates. In other material systems, the incompatibility of the crystallographic properties of the desired thin films and the substrate materials precludes epitaxial growth of the thin films. In such cases or, for example, in the case of epitaxially grown films where mesoscopic thickness are required, the desired thin films may be formed by detaching a surface layer of crystalline material from a bulk crystalline substrate material.
Co-assigned U.S. Pat. No. 6,120,592, U.S. Pat. No. 6,503,321 and U.S. Pat. No. 6,540,827, all of which are incorporated by reference herein in their entireties, describe methods using ion implantation and selective etching processes for detaching a thin film of high crystalline quality from bulk single crystal material (e.g., lithium niobate crystals). For convenience the methods disclosed in these patents are referred to herein as “conventional ion-slicing methods” or “CIS methods”. FIG. 1 schematically shows the primary processes used in the CIS methods. First, conventional ion-implantation processes are used to embed foreign ions (e.g., helium or hydrogen ions) in to a subsurface layer 120 of a single crystal bulk material 110 (e.g., LiNbO3). The embedded ions mechanically and/or chemically alter the structure of subsurface layer 120 making it susceptible to thermo-mechanical fracture or preferential chemical etching. Then, a super layer 130 of high crystalline quality material may be detached from the single crystal bulk by thermo-mechanically fracturing subsurface layer 120 or by chemically removing subsurface layer 120.
The detached thin films prepared by the CIS methods may be free standing. The thickness dimensions of the detached thin films (e.g., super-layer 130) and subsurface layer 120 are design parameters that can be controlled by choice of the type, energy and dose of the foreign ions that are implanted in the subsurface layer. The thickness of the detached thin films can be small—in the range a few microns to less than a micron. The small thickness makes the thin films fragile. Further, the freestanding thin films are prone to breakage while handling or processing in further device fabrication steps. A solution to avoid breakage of the fragile thin films may involve bonding super layer 130 to mechanical support substrates, prior to use. However, the bonding processes can be complex and are not always satisfactory. Additionally, further processing is required to define lateral portions of the bonded thin films that are actually used in a device structure.
Consideration is now being given to methods of forming thin film membrane structures having narrow lateral dimensions for direct use in device structures. Attention also is directed to electrical and optical device structures in which such membrane structures are advantageously used.